Abstract. The hepatitis C virus (HCV) has infected about 2 percent of the world's population and is a major cause of chronic liver diseases. Treatment of chronic hepatitis C has improved dramatically with the introduction of highly effective direct acting antivirals (DAA). At the same time, HCV transmission is increasing globally. In the United States, infection rates have doubled in many regions. Most of these new infections are undiagnosed and so new infections may occur at a faster rate than detection and treatment. There is an emerging consensus that a vaccine may be needed to prevent new HCV infections, and perhaps second infections in those cured with costly DAA. Considerable evidence supports a role for T cell immunity in protection from HCV persistence. Indeed, the only vaccine to enter phase I/II testing in humans incorporates only non- structural HCV proteins, with the objective of priming T cell immunity to prevent HCV persistence. There is still uncertainty, however, about mechanisms of CD4+ and CD8+ T cell failure during acute hepatitis C and the risk posed to vaccine development. The central hypothesis of the proposed research is that comparison of T cell responses in acute HCV infections will reveal mechanisms leading to exhaustion and persistence, or memory and life-long protection from chronic hepatitis C. Two aspects of the application are innovative. First, human liver biopsies are not medically justified during acute infection and so very little is known about intrahepatic T cell responses during this critical phase when infection outcome is determined. The proposed research will fill this gap in knowledge using archived blood and liver mononuclear cells from acutely infected chimpanzees with different infection outcomes. Second, cutting edge technologies to profile gene expression and TcR affinity have been adapted to study HCV-specific T cells. These technologies are innovative as they require very few HCV-specific CD8+ T cells to accomplish study goals. When combined with established flow cytometric methods, they will provide a comprehensive view of how transcriptional pathways, effector functions, and TcR affinity differ in resolving and persisting infections. These unique samples and technologies will be used to address two specific aims. Specific Aim 1 will identify early differences in HCV-specific CD8+ T cells associated with resolution versus persistence of infection. CD8+ T cells undergo functional exhaustion and select for HCV escape variants in infections that persist. Our objective is to understand how transcription factor networks, TcR affinity for antigen, and alterations in effector function contribute to this outcome. Specific Aim 2 is to identify mechanism(s) of CD4+ T cell failure in acute HCV infections that persist. CD4+ T cell help is essential for antiviral CD8+ T cell activity in HCV infection. There is to date no explanation for spontaneous loss of CD4+ T cell activity infections that persist, in part because frequencies in blood are too low for study. Here we will characterize CD4+ T cell effector functions and patterns of gene transcription in liver where the response is sequestered. These studies are expected to provide the first insight into reasons why CD4+ T cell fail, a key objective in understanding protection against HCV persistence relevant to vaccine development.